Organometallics 2007, 26, 4373-4375
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Communications Electropolymerized Pd-Containing Thiophene Polymer: A Reusable Supported Catalyst for Cross-Coupling Reactions Vincenzo G. Albano,† Marco Bandini,*,† Carolyn Moorlag,‡ Fabio Piccinelli,† Agostino Pietrangelo,‡ Simona Tommasi,† Achille Umani-Ronchi,† and Michael O. Wolf*,‡ Dipartimento di Chimica “G. Ciamician”, UniVersita` di Bologna, Via Selmi 2, Bologna, 40126 Italy, and Department of Chemistry, UniVersity of British Columbia, VancouVer, BC, V6T 1Z1, Canada ReceiVed March 7, 2007 Summary: Electrodeposition of Pd-containing oligothienyl complexes on porous graphite electrodes allows the preparation of a new, reusable heterogeneous organometallic catalyst for intraas well as intermolecular cross-coupling reactions. Palladium is one of the most utilized and versatile transition metals in modern synthetic organic chemistry, and a large number of transformations involve this noble metal as a catalyst or organometallic reagent.1 The scope of Pd-mediated crosscoupling reactions (i.e., Suzuki-Miyaura,2a Mizoroki-Heck,2b Sonogashira2c) continues to broaden,3 being commonly employed as bench test processes for new homo- and heterogeneous catalysts. As the chemical industry adopts Pd-based processes in manufacturing,4 demands for ecologically friendly transformations have moved research efforts toward new, easily handled, separable, and recoverable catalytic systems. To this purpose, anchoring or dispersion of an active catalyst complex onto an inert matrix is a commonly used strategy;5 however this often requires extensive synthetic steps, and contamination of reaction products by leaching of metal species is a concern.6 A relatively unexplored strategy to prepare thin films of organometallic catalysts is the controlled electropolymerization of metal-functionalized oligothiophenes.7 The direct electrochemical preparation of supported catalysts on electrodes avoids multistep synthetic pathways to introduce anchoring sites as well as spacers in the molecular skeleton. In addition, thin films may be readily deposited on high surface area electrodes such as * Corresponding authors. (M.B.) Tel: 39 51 2099544. Fax: 39 51 2099456. E-mail:
[email protected]. (M.O.W.) Tel: 604-822-1702. Fax: 604-822-2847. E-mail:
[email protected]. † Universita ` di Bologna. ‡ University of British Columbia. (1) Tsuji, J. In Palladium Reagents and Catalysts, New PerspectiVes for the 21th Century; Wiley: Chichester, England, 2004. (2) (a) Suzuki, A. Chem. Commun. 2005, 4759. (b) Beletskaya, I. P.; Cheprakov, A. V. Chem. ReV. 2000, 100, 3009. (c) Sonogashira, K. In MetalCatalyzed Cross-Coupling Reactions; Diederich, F., Stang, P. J., Eds.; Wiley-VCH: New York, 1998. (3) Metal-Catalyzed Cross-Coupling Reactions; de Meijere, A., Diederich, F., Eds.; Wiley-VCH: New York, 2004. (4) Corbet, J.-P.; Mignani, G. Chem. ReV. 2006, 106, 2651. (5) (a) Blaser, H.-U.; Indolese, A.; Schnyder, A.; Steiner, H.; Studer, M. J. Mol. Catal. A: Chem. 2001, 173, 3. (b) Sheldon, R. A.; van Bekkum, H. In Fine Chemicals through Heterogeneous Catalysis; Wiley-VCH: Weinheim, Germany, 2001. (c) Yin, L.; Liebscher, J. Chem. ReV. 2007, 107, 133. (6) Garret, C. E.; Prasad, K. AdV. Synth. Catal. 2004, 346, 899. (7) Wolf, M. O. AdV. Mater. 2001, 13, 545.
Scheme 1. Synthesis of [DAT3-Pd(η3-C3H5)][X] (2a: X ) BF4, 2b: X ) PF6).
porous graphite mesh. Thiophene-functionalized bipyridine ligands have been previously suggested as potential precursors of polymeric heterogeneous catalysts,8a and Pd catalysts have been attached as pendant groups to polypyrroles;8b however the incorporation of a catalytic center directly in the polymer backbone has not been explored. Our recent findings on the use of chiral diamino-oligothiophenes (DATs), as versatile ligands for catalysts,9 prompted us to consider the corresponding Pd complexes as suitable candidates for the electrochemical deposition of Pd-containing thiophene polymers on high surface area graphite electrodes.10 The electropolymerizable precursor complex [Pd(η3-C3H5)(1)][BF4] (2a) was prepared in 85% yield by reacting 19a (DAT3, 1 equiv) with [Pd(η3-C3H5)Cl]2 (0.5 equiv) in THF/CH2Cl2, followed by exchange with AgBF4. Spectroscopic characterization (ESI-MS, IR, UV/vis) and elemental analysis are consistent with the structure shown in Scheme 1. Although attempts to isolate suitable crystals of 2a for singlecrystal X-ray diffraction were unsuccessful, the molecular (8) (a) Electrochemical homo- and copolymerization of these systems was unsuccessful: Papillon, J.; Schultz, E.; Ge´linas, S.; Lessard, J.; Lemaire, M. Synth. Met. 1998, 96, 155. (b) These systems are not robust to multiple catalytic runs: Llobet, A.; Masllorens, E; Rodrı´guez, M.; Roglans, A.; BenetBuchholz, J. Eur. J. Inorg. Chem. 2004, 1601. (9) (a) Albano, V. G.; Bandini, M.; Melucci, M.; Monari, M.; Piccinelli, F.; Tommasi, S.; Umani-Ronchi, A. AdV. Synth. Catal. 2005, 347, 1507. (b) Albano, V. G.; Bandini, M.; Barbarella, G.; Melucci, M.; Monari, M.; Piccinelli, F.; Tommasi, S.; Umani-Ronchi, A. Chem.-Eur. J. 2006, 12, 667. (c) Bandini, M.; Piccinelli, F.; Tommasi, S.; Umani-Ronchi, A.; Ventrici, C. Chem. Commun. 2007, 616. (10) (a) Clot, O.; Wolf, M. O.; Patrick, B. O. J. Am. Chem. Soc. 2000, 122, 10456. (b) Pozo-Gonzalo, C.; Berridge, R.; Skabara, P. J.; Cerrada, E.; Laguna, M.; Coles, S. J.; Hursthouse, M. B. Chem. Commun. 2002, 2408. (c) Mucci, A.; Parenti, F.; Pigani, L.; Seeber, R.; Zanardi, C.; Pilo, M. I.; Spano, N.; Manassero, M. J. Mater. Chem. 2003, 13, 1287. (d) Clot, O.; Wolf, M. O.; Patrick, B. O. J. Am. Chem. Soc. 2001, 123, 9963.
10.1021/om070210l CCC: $37.00 © 2007 American Chemical Society Publication on Web 08/02/2007
4374 Organometallics, Vol. 26, No. 18, 2007
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Figure 1. Coordination modes of the Pd(η3-C3H5) unit to substituted trans-1,2-diaminocyclohexane.
Figure 2. X-ray molecular structure of [Pd(η3-C3H5)(1)]+ as a [PF6]- salt (2b). The allyl ligand is disordered, and the dominant orientation (55%) is shown. Selected bond lengths (Å): Pd(1)-N(1) 2.110(5), Pd(1)-N(2) 2.101(5), Pd(1)-C(33) 2.137(9), Pd(1)-C(35) 2.199(8), Pd(1)-S(2) 3.306(2).
structure of the analogous [Pd(η3-C3H5)(1)][PF6] complex 2b (Figure 2) could be obtained. The structure shows a N,Nchelating cycle in an envelope conformation with the nitrogen atoms showing opposite configurations [N(1)(R) and N(2)(S)]. A close intramolecular S(2)‚‚‚Pd(1) contact [(3.306(2) Å)] is observed similar to that reported in the analogous DAT2-Pd complex [(3.340(1) Å)].9a 1H
NMR studies of 2a were carried out to gain insight into the behavior of the cationic Pd complex in solution. The coordination of the Pd(η3-C3H5)unit to the substituted trans1,2-diaminocyclohexane gives a five-membered metallacyclic ring in which the nitrogen atoms are locked in a tetrahedral environment. In 2a three possible stereoisomers at the nitrogen centers (two dl and meso isomers) can be formed (Figure 1).11a,b Three distinct sets of allylic protons were observed in the 1H NMR spectra in both CDCl3 (-80 f 35 °C) and CD3CN (-45 f 70 °C) over the entire temperature range studied. It has been shown that in strongly coordinating solvents such as CD3CN the η1 and η3 allyl isomers rapidly equilibrate;11c,d thus we attribute the observed sets of allyl protons to the two dl isomers and the meso species (ratio at room temperature: 1:1.15:1.73). In CDCl3 similar results are observed, suggesting that even in this less coordinating solvent rapid equilibration of the η1 and η3 allyl species occurs, possibly due to interaction between the Pd center and the ancillary thienyl groups,12 as observed in the (11) (a) Perica`s, M. A.; Puigjaner, C.; Riera, A.; Vidal-Ferran, A.; Go´mez, M.; Jime´nez, F.; Muller, G.; Rocamora, M. Chem.-Eur. J. 2002, 8, 4164, (b) Rafii, E.; Dassonneville, B.; Heumann, A. Chem. Commun. 2007, 583. (c) Hansson, S.; Norrby, P.-O.; Sjo¨gren, M. P. T.; Åkermark, B.; Cucciolito, M. E.; Giordano, F.; Vitagliano, A. Organometallics 1993, 12, 4940. (d) Canal, J. M.; Go´mez, M.; Jime´nez, F.; Rocamora, M.; Muller, G.; Dun˜ach, E.; Franco, D.; Jime´nez, A.; Cano, F. A. Organometallics 2000, 19, 966. (12) Oslinger, M.; Powell, J. Can. J. Chem. 1973, 59, 274.
Figure 3. Cyclic voltammogram of 2a on a Pt electrode.
solid-state structure of 2b. The UV/vis spectrum of 2a has λmax ) 360 nm, close to the maximum absorption of terthiophene (355 nm).13 Complex 2a was electropolymerized on a Pt electrode (Figure 3). Electropolymerization of related Pd complexes has previously been shown to occur via coupling of the R-positions of the terminal thiophene rings.14 For catalytic studies, films were electropolymerized on carbon paper (Toray TGP-H-030) by cycling from 0 V to the potential where polymerization starts (10 cycles typically used). The amount of poly-2a on the carbon films is difficult to accurately determine, but mass changes of
